4 Dispersion or Orowan strengthening One of the great successes of dislocation theory is Orowan's mechanism for the interaction between glissile dislocations and hard, nonshearable precipitates. Under an applied stress, a dislocation pinned by two particles in its glide plane bows out. At a critical stress, adjacent bowed segments contact and join, bypassing the precipitate and leaving a dislocation shear loop around the bypassed particle. The increase in the flow stress required to bypass particles of interparticle spacing h is t % tf  k Gb r 4 where t is the macroscopic flow stress and tf is the stress required for dislocation motion in an otherwise obstacle-free crystal.

Casting). Dispersion-strengthened materials are briefly considered at the end of this chapter. 5 Work hardening Because of the stress field around each dislocation, the motion of dislocations through the lattice is impeded by the presence of other dislocations. This is responsible for the workhardening effect in metals. A high work-hardening rate is important in many applications because it increases cold-forming ability and the work of fracture. Metals like Cu, which have a low stacking fault energy, work-harden more rapidly than metals like Al because the widely spaced partial dislocations are more prone to forming sessile (nonmobile) dislocation segments.

Li et al. (1995) measured an increase in yield and ultimate tensile strength of a particulate- Effects of Dislocations on Mechanical Properties reinforced aluminum MMC by dropping the temperature from ambient to liquid nitrogen temperature, and concluded that this cryogenic strengthening was due to both the expected strength increase of metals at low temperature and to dislocation hardening from thermal mismatch. Since plastic flow is not fully reversible, a low temperature excursion can change ambient temperature properties.